The effect of viral infections upon delayed hypersensitivity in man and in experimental animals is being investigated using M-M-R vaccine (mumps-measles-rubella vaccine) in humans and distemper virus vaccine infections in dogs. Administration of these agents results in a loss of skin reactivity to previously reactive skin test antigens. The enumeration and function of T lymphocytes is being investigated to determine the mechanism of the viral inhibition of existing delayed hypersensitivity. At present, it seems likely that there is a suppressive effect upon existing T cells rather than an infection by virus of T cells with an absolute reduction in number.